N-Methylacetamide/water clusters in a hydrophobic solvent

Abstract

N-methylacetamide (NMA) and water structures in the hydrophobic solvent carbon tetrachloride are studied by simple FTIR spectroscopy. At very low concentrations of NMA and water mainly the monomer species of both systems are present. This is indicated by two NH stretch frequencies of the trans (ν = 3475 cm⁻¹) and cis-NMA monomer (ν = 3431 cm⁻¹) and the symmetric and asymmetric frequencies of the water monomer (ν₃ = 3619 cm⁻¹ and (ν₁ = 3712 cm⁻¹) accompanied by their librational contributions. By increasing the NMA concentration first dimer structures are formed: pure NMA dimers and two NMA/water dimers, with water molecules attached to the CO or to the NH group of NMA. Additionally larger NMA oligomers are formed. Temperature and concentration dependent measurements along with density functional calculations allowed the assignment of all structures. Some clues about the cluster stability and their formation process are obtained. The results give also some hints for the competition between peptide–peptide and water–peptide hydrogen bonds which are important for protein folding.